1. Field of the Invention
The present invention relates to an image forming apparatus that transfers a toner image to a recording medium to form an image. Particularly the invention relates to control in which, in order to enhance the image formation, a distortion of the toner image is corrected to transfer the toner image to the recording medium.
2. Description of the Related Art
In an electrophotographic system image forming apparatus, the toner image on a photosensitive drum or an intermediate transfer belt, which is of an image bearing member, is transferred to a recording material to form the image.
Usually, in the image forming apparatus, the control is performed such that a time the toner image reaches a transfer portion is matched with a time the recording material reaches transfer portion. The toner image on the photosensitive drum or intermediate transfer belt, which is of the image bearing member, partially expands or shrinks by a speed fluctuation of the photosensitive drum or intermediate transfer belt. When the expanded or shrunk toner image is directly transferred, the expansion or shrinkage of the image remains on the recording material. Therefore, for example, Japanese patent Application Laid-Open No. 2004-78016 discloses a technique of solving the problem.
In the technique disclosed in Japanese patent Application Laid-Open No. 2004-78016, a position detecting pattern formed in the intermediate transfer belt is detected, and a speed of the intermediate transfer belt is determined from the time a leading end of the position detecting pattern passes, the time a rear end of the position detecting pattern passes, and a distance between the leading end and the rear end. On the other hand, a leading end and a rear end of the recording material are detected, and a speed of the recording material is determined from the time the leading end of the recording material passes, the time the rear end of the recording material passes, and a length of the recording material. The driving of the intermediate transfer belt and the driving of the recording material are controlled according to the speed of the intermediate transfer belt and the speed of the recording material.
However, only the average speed is determined in the technique disclosed in Japanese patent Application Laid-Open No. 2004-78016. Therefore, not only the control is hardly performed according to the minute, small speed fluctuation, but also the partial expansion or shrinkage of the image is hardly corrected.
The following methods are disclosed in order to solve the problems in a tandem type image forming apparatus, in which plural image bearing members having different development colors are arranged along a conveying member (such as an intermediate transfer belt and a recording material conveying belt) to form a full-color image.
That is, in the control technique with high accuracy, a recording layer is provided in each of a first photosensitive drum and a second photosensitive drum, and a first position index and a second position index are recorded in synchronization with write of a scanning line. On the other hand, a recording layer is also formed in the intermediate transfer belt, and a conveying member position index is recorded in the intermediate transfer belt every time the first position index is sensed in a primary transfer portion of the first photosensitive drum.
In the primary transfer portion of the second photosensitive drum, a determination of the advance or delay of the corresponding second position index is made every time the conveying member position index is read, and a rotating speed of the second photosensitive drum or a position along a rotating direction of the intermediate transfer belt is adjusted to superimpose a first color toner image and a second color toner image.
There is another technique of controlling the speed of the photosensitive drum to align the positions of the toner images (for example, see Japanese patent Application Laid-Open Nos. 2004-145077, 2003-173091, and 2006-301007).
In the conventional techniques, the color toner images can accurately be superimposed in the transfer portion. At the same time, disadvantageously the partial expansion or shrinkage of the toner image is enlarged because of the adjustment of the photosensitive drum.